Uniform polarity resin electrostatic toners

ABSTRACT

A UNIFORM POLARITY, PARTICULATE, DYED RESIN ELECTROSTATIC TONER IS PREPARED BY DISSOLVING AT LEAST ONE MONO- OR DIFUNCTIONAL ORGANIC ACID NIGROSINE SALT IN A THERMOPLASTIC RESIN BINDER. ADMIXING THE COMPOSITE TONER WITH A CARRIER SUCH AS POWDERED IRON PRODUCES A DEVELOPER FOR ELECTROSTATIC LATENT IMAGES.

US. Cl. 252-621 Claims ABSTRACT OF THE DISCLOSURE A uniform polarity, particulate, dyed resin electrostatic toner is prepared by dissolving at least one monoor difunctional organic acid nigrosine salt in a thermoplastic resin binder. Admixing the composite toner with a carrier such as powdered iron produces a developer for electrostatic latent images.

This application is a continuation-in-part of US. patent application Ser. No. 736,552 filed June 13, 1968 and now abandoned.

The present invention relates to photography and particularly to electrophotography and the dry development of electrostatic latent images.

Electrophotographic elements or materials conventionall involve an electrically conducting support on which is coated a photoconductive insulating material. After overall charging such as by a corona source an an imagewise light exposure that discharges the photoconductor in the exposed areas, a latent electrostatic image remains. This latent electrostatic image, as well as latent electrostatic images produced by other techniques, can be rendered visible by treatment with an electrostatic developing composition or developer. Conventional dry developers include a carrier that can be either a magnetic material such as iron filings, powdered iron or iron oxide, or a triboelectrically chargeable, non-magnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium chloride. As well as the carrier, electrostatic developers include a toner that is usually a resinous material suitably darkened for image viewing purposes with a colorant like dyestuffs or pigments such as carbon black.

To develop a latent electrostatic image, the dry developer is applied to an imagewise electrostatically charged surface by techniques such as the magnetic brush means described in US. Pat. No. 3,003,462. The developer composition, including both toner and magnetic carrier particles, is maintained, during the development cycle, in a loose, brushlike orientation by a magnetic field surrounding, for example, a rotatable non-magnetic cylinder having a magnetic means fixedly mounted inside. The magnetic carrier particles are attracted to the cylinder by the described magnetic field, and the toner particles are held to the carrier particles by virtue of their opposite electrostatic polarity. Before and during development the toner acquires an electrostatic charge of a sign opposite to that of the carrier material due to triboelectric charging derived from their mutual frictional interaction.

When this brushlike mass or magnetic brush of carrier and toner particles is drawn across the photoconductive surface bearing the latent electrostatic image, the toner particles are electrostatically attracted to an oppositely charged laent image and form a visible toner image corresponding to the latent image. If both charges are of a like sign, then the toner particles adhere to the photoconductors discharged areas. In such fashion, it is possible to form either positive or negative reproductions.

Patented Mar. 7, 1972 In developing a latent electrostatic image, however, conventional toners tend to contain a number of particles having either a polarity opposite to that desired or a diminished net charge, although of the correct polarity. Such an occurrence derives from the varying triboelectric cherge characteristics of the toners resin binder and colorant, and it promotes the deposition of toner particles in background are-as and also contributes to an undesirably weak attraction between toner and carrier and thereby promoting the formation of an image having either irregular density or other defects such as streaks etc. Additionally, when development is by well-known magnetic brush techniques, non-uniformly charged toner particles tend to be thrown off a rotating magnetic brush and are not then available for image development.

Additionally, it is often conventional practice to place a bias voltage of a polarity opposite to that of the toner particles across the magnetic brush. Such action provides additional attraction between toner particles and the magnetic brush apparatus and thereby tends to restrict background toning in exposed areas of the photoconductor or in the background area of any other insulating surface bearing a latent electrostatic image still possessing a residual charge. The bias voltage is not so great, however, as to interfere with the toning of desired image areas having a sufiicient charge to attract and draw away toner particles from the carrier.

Where certain of the toner particles have an opposite polarity, as in non-uniform polarity toners, such particles are repelled from the magnetic brush by this bias voltage. In like fashion the intended image areas of the charged insulating surface, having a polarity similar to the opposite polarity toner particles, also exerts a repelling force with the resulting deposition of opposite polarity toner particles in nonimage or background areas.

Accordingly, it is an object of this invention to provide, for electrophotographic development purposes, a new uniform-polarity toner.

It is another object of this invention to provide, for electrophotographic development purposes, a novel uniform-polarity toner having an increased resistance to background development.

An additional object of the present invention is to provide, for electrophotographic development purposes, a new uniform-polarity toner that promotes developed images of an increased and uniform density.

Still an additional object of this invention is to provide, for electrophotographic development purposes, a novel uniform-polarity toner that is resistant to magnetic brush throwoif.

Yet other objects of the instant invention will become apparent from a' consideration of the specification and appended claims.

The objects of this invention are accomplished with a uniform polarity, dyed resin electrostatic toner comprising a thermoplastic resin having uniformly dissolved therein a coloring amount of a nigrosine salt selected from the group consisting of nigrosine salts of mono and difunctional organic acids having from about 2 to about 26 carbon atoms.

The subject uniform-polarity toners incorporate a polymeric, thermoplastic resin in which is uniformly dissolved a nigrosine salt colorant that is soluble in the resin. The term uniform-polarity refers to the presence of toner particles having only plus or minus polarity, without particles of opposite sign. In conventional dry toners, carbon black and other pigments that normally acquire a negative triboelectric charge are typically employed as dispersed :colorants, whereas the binders in which such pigments are dispersed generally acquire a positive polarity. The composite toner particle, comprised of both the dispersed colorant and resin dispersing medium, can have a surface that is either predominantly resin binder or pigment. The net charge of such a toner particle is dependent on its surface character, and is proportional to the respective surface exposure of either dispersed colorant or resin binder. The net surface charge on a toner particle can be conveniently represented by the equation 2Q=qa+q a where q is the negative charge per unit of colorant surface area, a is the total colorant surface area, q, is the total positive charge per unit of resin surface area and a is the total resin surface area. As such, conventional dry toners typically have toner particles of both signs, since it is difiicult to uniformly disperse a colorant such as carbon black at the relatively low concentrations used in toner formulations, and such toners are subject to the previously noted disadvantages such as background toning. Similar detrimental characteristics are possessed of toners incorporating a dyestuff colorant that is insoluble in the resin binder. The uniform-polarity toners of this invention, however, are a common solution of colorant and resin, and it is possible to form toner particles that possess substantially uniform surface characteristics and a net charge of like sign, thereby avoiding the noted disadvantages inherent in dry toners having as colorants either dispersed pigments or dyes that are insoluble in the resin binder.

The thermoplastic resins useful in the practice of the present invention can be used alone or in combination and include those resins conventionally employed in electrostatic toners and in which the subject nigrosine salts are soluble. Exemplary of such resins are polystyrenes, rosin modified maleic alkyd resins, polyamides, polyester condensates like poly(ethylene terephthalate-isophthalate) and poly(ethylene glycol-neopentylglycol-terephthalate-isophthalate), modified alkyd resins and the like thermoplastic resins.

Nigrosine salts that are advantageous herein include nigrosine salts of mono and difunctional organic acids. Exemplary of such organic acids are aliphatic and aromatic, monoand dicarboxylic acids typically having from about 2 to about 26 carbon atoms such as chloroacetic acid, stearic acid, docosanoic acid, sebacic acid, lauric acid, azelaic acid, adipic acid, abietic acid and the like. The subject nigrosine salts like nigrosine stearate, nigrosine sebacate, etc., are conveniently prepared merely by admixing nigrosine free base, Colour Index No. 50415B, and substantially a neutralizing amount of at least one organic acid, such as those described herein, at a temperature above the melting point of each ingredient. If desired, a moderate excess of acid or free base can be used; conventionally, the excess component is not usually greater than about 75% based on the respective number of equivalent weights.

The composite toner particles, including both nigrosine salt and thermoplastic resin, conventionally have a melting point between about 60 C. and about 275 C., which are temperatures typically experienced in electrostatic development to heat stabilize the toner image by fusing it to a support. The more elevated fusion temperatures are conventionally used in high-speed operations where exposure to an energy source is of short duration. Wider ranging melting points are advantageously used, however, in accordance with conventional practice for particular electrostatic development situations. Generally, each of the nigrosine salt and the thermoplastic resin have a meling point within the range desired for the particular toner composition, although it is possible to vary the melting point of either the nigrosine salt or the resin binder beyond the preferred range, while maintaining the melting point of the composite toner at an advantageous temperature level. For example, a nigrosine salt whose melting point is in excess of the preferred upper limit noted herein can be dissolved in a lower melting thermoplastic resin to provide a uniformpolarity toner having desirable melt characteristics. In

addition to the nigrosine salt colorant, small amounts of color-balancing dyes can be employed in conjunction therewith to achieve an additionally neutral coloration in the ultimate toner composition. Exemplary of such colorbalancing dyes are 1,4 dihydroxy 5,8 bis(4-methylanilino) 9,10 anthraquinone and 4-(4-methyl-2-nitrophenylazo) S methyl 3 pyrazolone. It is preferred, however, that the color-balancing dyes do not substantially alter the triboelectric chargeability of the toner so as to deleteriously affect the electrostatic attraction between toner and carrier in the composite developer.

In the subject toners, the nigrosine salts are typically present in coloring amounts that generally range from about 3% to about 8% by weight, with the thermoplastic resin forming the remaining portion of from about 92% to 97% by weight of the composite toner particle. Additionally, small amounts of color-balancing dyes can also be dissolved in the resin binder if desired for the production of a neutral black color. Conventionally, such color balancing dyes are included in amounts less than about 1.5% of the toners total weight, although greater amounts can be used if desired as long as the triboelectric properties of the toner are preserved.

The instant uniform polarity toners can be prepared by alternate procedures. Two convenient techniques are spray-drying and melt dispersion. Spray-drying comprehends dissolving the nigrosine salt (previously prepared by admixing nigrosine free base and an organic acid at a temperature above the acids melting point) and the thermoplastic resin in a volatile organic solvent such as dichloromethane. This solution is then sprayed through an atomizing nozzle using a substantially non-reactive gas such as nitrogen as the atomizing agent. During atomization, the volatile solvent evaporates from the airborne droplets, producing toner particles of the uni formly dyed resin. The ultimate particle size is determined by varying the size of the atomizing nozzle and the pressure of the gaseous atomizing agent. Conventionally, particles of a diameter between about 2p. and about 25;; are used, with particles between about 5,11. and 15 being preferred, although larger particles can be used where desired for particular development conditions or developer compositions.

When one of the uniform-polarity toners of this invention is prepared by melt dispersion techniques, a melt is prepared from either the nigrosine salt and the thermoplastic resin or from nigrosine free base, an organic acid such as those described herein and the thermoplastic resin. In each case, the melt temperature is maintained at a temperature above the melting points of the individual ingredients, to promote complete reaction and dissolution but suitably below that temperature at which degradation of the resin binder and colorants might occur. Typically, temperatures of from about 50 C. to about 200 are used, although wider variations can be employed where desired.

The melt is stirred or otherwise blended such as on resin compounding rolls to promote complete mixing of the resin and dissolved nigrosine salt colorant, after which the mixture is cooled and solidified. The resultant solid mass of toner is then ground to produce small toner particles, preferably having a diameter less than about 25- 1. although larger particles can be use if desired.

The toner, prepared by a method such as those described above is then admixed with a suitable particulate carrier such as powdered iron, magnetic iron oxide, glass beads and many other well-known conventional carriers to produce a composite electrophotographic developer. Conventionally, the toner and carrier are present in such developer in amounts by weight varying from about 93% to about 99% carrier and from about 1% to about 7% toner, but more extensive variations can be used where desired.

The developer can then be employed to form visible images corresponding to latent electrostatic images. Image development is typically accomplished by contacting a photo conductive or other electrically insulating surface bearing a latent electrostatic image with the developer composition, whereupon the toner particles are electrostatically attracted thereto. A conventional means of obtaining imagewise deposition of toner particles is the magneticbrush technique described hereinabove. Another typical development means is cascade development wherein toner and carrier are poured over or otherwise contacted against the element bearing a latent electrostatic charge image. Subsequent to development the toner image can be fixed or rendered permanent by heating to melt the thermoplastic resin and thereby fusing the toner to the photoconductive sheet. Alternatively, the toner can be transferred to a receiver sheet by means such as contact transfer and then fixed by similar means in situ on the receiver sheet. Also, the toner image can be fixed by solvent treat ment, whereupon the toner is made to adhere to asupporting surface by a brief exposure to a solvent or solvent vapors that partially dissolve the toners resin component and cause it to become adhered to the support.

derstanding of the invention.

EXAMPLE 1 Two parts, by weight, of stearic acid are reacted with one part of nigrosine free base (62.1% stearic acid based on the respective number of equivalent weights) at the melting point of the stearic acid (approximately 69-70 C.). Eighteen grams of the resulting nigrosine stearate are added to 150 grams of a low molecular weight polystyrene marketed under the designation Piccolastic D 125 by the Pennsylvania Industrial Chemical Corp. and 50 grams of a rosin modified maleic alkyd resin marketed under the designation Amberlac D 96 by Rohm & Haas Co., and this mixture is stirred at a temperature of 135 C. in a breaker heated with an electric heating mantle. After the nigrosine stearate is thoroughly dissolved, the melt is cooled and solidified, then ground in a disc grinder so that the particles pass through the perforations of a -mesh screen. Fine grinding to a toner particle size less than in diameter is accomplished with a fluid energy mill (model Gem T-X by G. W. Helme Co., Inc., Trost Jet Mill Division) at a grinding pressure of p.s.i.g and a feed rate of 476 g./hr. The resulting toner is then added to finely divided iron particles of a 60-1-20 mesh size and marketed by the Glidden Co., to prepare an electrostatic developer composition containing 3% toner by weight. A photoconductive sheet is then charged, exposed to a line image and imagewi se developed by brushing the photoconductor having a latent electrostatic image with the developer composition held on a manually operated magnetic brush having a 50 volt negative bias. The toner image formed on the electrostatic image is then transferred to a white paper receiver sheet by contact transfer and is fused thereon in an imagewise fashion to provide a stable, permanent copy.

EXAMPLE 2 Nigrosine base in the amount of 9 grams and 10 grams stearic acid (48% stearic acid based on the respective number of equivalent Weights) are added to a resin melt as described in Example 1. This mixture is blended at 135 C. as in Example 1 and is similarly solidified and ground to prepare a uniform-polarity toner, except that grinding is carried out at a feed rate of 67.9 g./hr. After preparing a developer composition like that described in Example 1, a latent electrostatic image is toned and the toner image transferred and fixed by fusion all as in Example 1 to provide a dense, stable image comparable to that of Example 1.

EXAMPLE 3 A toner and developer are prepared and used as described in Example 1 to develop a latent electrostatic image, except that only 5 grams stearic acid (30.2%

stearic acid based on the respective number of equivalent weights) is added to the resin, and that the feed rate used in the grinding operation is 76.5 g./hr. The image obtained is of a quality comparable to those of Examples 1 and 2.

EXAMPLE 4 To a resin melt as described in Example 1 is added 9 grams nigrosine base containing 4.25 meq. of base per gram of solid and 10.88 grams stearic acid which is sufficient stearic acid to exactly neutralize the base. The color of this melt is a reddish-blue. A neutral hue is then obtained by adding 1 gram each of the color-balancing dyes 1,4 dihydroxy 5,8 bis-(4-methylanilino)-9,10-anthraquinone and 4 (4 methyl 2 nitrophenylazo)- 5 methyl 3 pyrazolone. A toner and developer are prepared as described in Example 1, and an electrostatic latent image is toned, transferred and fixed in a like fashion to provide a dense, stable copy without undesirable deposition of toner particles in background areas. Portions of the toner are used to develop an additional latent electrostatic image by mechanically operated magnetic brushes. Two brushes are used, one having a developer With 2% toner by weight and the other with 5% toner by Weight. They are rotated at r.p.m., with a 70 volt negative bias. The image so obtained is dense and sharp. with clean background areas.

EXAMPLE 5 A toner is prepared as in Example 1 except that the 9 grams nigrosine base is used without the addition of stearic acid. Additionally, the grinding feed rate is 180.2 g./hr. This toner is then used to prepare a developer as described in Example 1, and a latent electrostatic image is then toned with this developer, transferred and fixed by fusion according to the procedure of Example 1. Image sharpness is good, but image density is decreased and some background deposition of toner particles occurs in nonimage background areas.

EXAMPLE 6 To 26 grams of the thermoplastic polystyrene resin of Example 1 and 8.7 grams of the rosin modified maleic alkyd resin of Example 1 is added 2.7 grams of the stearic acid-nigrosine base reaction product described in Ex ample 1. These ingredients are then dissolved in 452 ml. dichloromethane, and the resulting solution is spray dried to form toner particles of a diameter less than 25y. using nitrogen as the atomizing gas. Using this spray-dried toner and according to the procedure of Example 1, a developer is prepared and a latent electrostatic image is toned, providing a copy having a densely colored image with clean background areas that are free of deposited toner particles.

EXAMPLE 7 A toner is prepared using the ingredients described in Example 4, blending being carried out on resin compounding rolls at a roll temperature of C. After cooling to solidify the melt, 221 grams of the mixture is dissolved in 2680 ml. of dichloromethane and spray dried as in Example 6. The toner so prepared is then used, according to the procedure of Example 1, to prepare an electrostatic developer which in turn is employed to tone a latent electrostatic image by means of a manually operated magnetic brush having a 60 volt negative bias. After contact transfer and image stabilization by fusion, the copy corresponding to the latent electrostatic image has a dense, sharp image with toner-free background areas.

EXAMPLE -8 To grams of the thermoplastic polystyrene resin and 50 grams of the rosin modified maleic alkyd resin described in Example 1 is added 9 grams of nigrosine base and 11.0 grams of abietic acid (48.8% abietic acid based on equivalent weights). Also added is 1 gram each of the two color-balancing dyes described in Example 4. Blending is accomplished as described in Example 1, except that the melt temperature is elevated to insure melting of the abietic acid (M.P. 137-166 C. var.). Grinding is also like that of Example 1, but with a feed rate of 122 g./ hr. A developer is prepare and an image produced according to the procedure of Example 1 except that a 50-volt negative bias is applied to the manually operated magnetic brush, with the stable copy having a dense, sharp image area and a non-image background area free of deposited toner.

EXAMPLE 9 Seven toners (ag) are prepared according to the procedure of Example 4, except that equivalent amounts of other organic acids (sufficient to exactly neutralize 9 grams of the particular nigrosine base having 2.28 meq. base per gram of solid) are substituted for the stearic acid as follows:

Grams (a) lauric acid 4.10 (b) azelaic acid 1.92 (c) sebacic acid 2.06 (d) adipic acid 1.50 (e) abietic acid 6.19 (f) chloroacetic acid 1.94 (g) docosanoic acid 7.10

Additionally, blending is carried out at melt temperatures sufficient to obtain complete solubilization of the ingredients. Using the described toners, electrostatic developers are prepared and electrostatic latent images are toned, all according to the procedure of Example 4. The images obtained with each of toners a-f are dense and sharp, with no deposition of toner particles in non-image background areas.

EXAMPLE 10 T illustrate the present invention, four toners (a d) are prepared as described in Example 1 with the exception of the nigrosine compoundings which are as follows.

Nigrosine base 9 Stearic acid 10.9

(b) Nigrosine base 9 (c):

Spirit soluble nigrosine 9 Stearic acid 10.9 (d) Spirit soluble nigrosine 9 From each toner, an electrostatic developer is prepared according to the procedure of Example 1. A weighed portion of each of the developers is placed in an iron tube that is covered at each end with a 200 mesh screen that retains all carrier particles within the tube. An air stream 'is then directed through the tube, blowing toner particles off the carrier, through the 200 mesh screen at the exit end and into a Faraday cage condenser that is at an initial electrical potential of zero volts as measured by an electrometer. As the triboelectrically charged toner particles settle on the walls of the cage, their electrical potential is measured by the electrometer. The potential obtained is converted to electrical charge in microcoulombs and this figure is divided by the weight in grams of each toner that has settled in the Faraday cage, providing the net toner charge in microcoulombs per gram. Since the net charge is the algebraic sum of the charges on each toner particle, 21 higher reading is indicative of a higher degree of uniform polarity. For each of the four toners prepared as described in this example, the charge per gram is as follows:

p.c./g.

Toner a 10.7

Toner b 7.0

Toner c 4.6

Toner d 3.0

The wide disparity in charge intensity per unit mass illustrates the advantageous uniform polarity of the subject toners.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. An electrostatic developing composition containing a particulate carrier and a toner composition, said toner composition exhibiting uniform-polarity and comprising a particulate, dyed thermoplastic resin having dissolved therein an amount effective to color said resin of a nigrosine salt consisting essentially of a reaction product of nigrosine base, Colour Index No. 50415 B, and at least one organic acid selected from the group consisting of monoand dicarboxylic acids having from about 2 to about 26 carbon atoms.

2. An electrostatic developing composition according to claim 1, the toner composition comprising from about 92 percent to about 97 percent by weight of said thermoplastic resin and from about 3 percent to about 8% by weight of said nigrosine salt.

3. An electrostatic developing composition according to claim 1 wherein the thermoplastic resin is selected from the group consisting of polyamide resins, polystyrene resins, rosin modified maleic alkyd resins, polyester resins and mixtures thereof.

4. An electrostatic developing composition according to claim 1 wherein the organic acid is selected from the group consisting of aliphatic and aromatic, monoand dicarboxylic acids having from 2 to about 26 carbon atoms.

5. An electrostatic developing composition according to claim 2 wherein said organic acid is selected from the group consisting of chloroacetic acid, adipic acid, azelaic, sebacic acid, lauric acid, stearic acid, docosanoic acid, abietic acid and mixtures thereof.

6. In an electrostatic developing composition containing a particulate carrier and a toner composition, the improvement wherein said toner exhibits uniform-polarity and comprises a particulate, dyed thermoplastic resin selected from the group consisting of polyamide resins, polystyrene resins, rosin modified maleic alkyd resins and mixtures thereof having dissolved uniformly therein from about 3 percent to about 8 percent by weight of a nigrosine salt consisting essentially of a reaction product of nigrosine base, Colour Index No. 50415B, and at least one organic acid selected from the group consisting of chloroacetic acid, adipic acid, azelaic acid, sebacic acid, lauric acid, stearic acid, docosanoic acid, abietic acid and mixtures thereof.

7. In an electrostatic developing composition containing a particulate carrier and a toner composition, the improvement wherein said toner composition exhibits uniform-polarity and comprises from about 92 percent to about 97 percent by weight of a particulate, dyed thermoplastic resin of polystyrene in admixture with a rosin modified maleic alkyd resin and from about 3 percent to about 8 percent by Weight of nigrosine stearate.

8. An electrostatic developing composition according to claim 7 wherein said polystyrene resin and said rosin modified maleic alkyd resin are present in a weight ratio of about 3:1.

9. In an electrostatic developing composition containing a particulate carrier and a toner composition, the improvement wherein said toner composition exhibits uniformpolarity and comprises from about 92 percent to about 97 percent by Weight of a thermoplastic resin comprising a polystyrene resin in admixture With a rosin modified maleic alkyd resin and from about 3 percent to about 8 percent by weight of nigrosine docosonoate.

10. An electrostatic developing composition comprising from about 1 percent to about 7 percent by weight of a toner composition and from about 93 percent to about 99 percent by weight of a carrier vehicle selected from the group consisting of powdered iron, magnetic iron oxide and glass beads, said toner composition exhibiting uniform-polarity and comprising from about 92 percent to about 97 percent by weight of a thermoplastic resin selected from the group consisting of polyamide resins, polystyrene resins, rosin modified maleic alkyd resins and mixtures thereof having uniformly dissolved therein from about 3 percent to about 8 percent by weight of a nigrosine salt consisting essentially of a reaction product of nigrosine base, Colour Index No. 50415B, and at least one organic acid selected from the group consisting of chloroacetic acid, adipic acid, azelaic, sebacic acid, lauric acid, stearic acid, docosanoic acid, abietic acid and mixtures thereof.

References Cited UNITED STATES PATENTS OTHER REFERENCES Ghosh et al. Journal Indian Chem. Soc. vol. 41, No. 8, 1964, pages 567-572.

Condensed Chemical Dictionary, seventh edition, 1966,

15 page 664.

Hackhs Chemical Dictionary, 4th edition, page 814.

Color Index Code, page 3420.

Kingzetts Chem. Encyclopedia, 9th edition 1960, page 645.

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Assistant Examiner PO-ww M WEED STATE PATENT mm iwm'm 0F EQH Patent No. 3 6 +7 .696 Dated March 7., 1.972

Inventor) James R. Olson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8 line M3, 2" should read Signed and sealed this 3rd day, of April 1973.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. I ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

